A Co-Star Luma is an orange Luma that appears in Super Mario Galaxy 2 as a playable character and in the game's logo. The Co-Star Luma appears alongside Mario in the cooperative two-player mode known as Co-Star Mode, in which the second player is able to control it using the Wii Remote. Once it appears, it will follow behind Mario in the same manner as one of the mini-clouds that appear when the Cloud Flower is in use. If the Cloud Flower is actually in use, the Co-Star Luma will always follow Mario behind the last mini-cloud that trails Mario's head, as opposed to directly behind him like it normally would.

The Co-Star Luma can perform moves such as spinning, and, in addition to collecting and shooting Star Bits just like the first player can, the Co-Star Luma can also perform other actions that a single player cannot, such as temporarily immobilizing certain enemies or barriers and picking up far-away objects, including 1-Up Mushrooms, air bubbles (when underwater), and coins, which the Co-Star Luma will then bring back to Mario. The Co-Star Luma can also perform a variety of other actions, including activating switches or other objects; touching Spiky Plants, causing them to retract their spikes; and spinning Lumas (excluding Lubba and Hungry Lumas) or Luigi to receive Star Bits. A Co-Star Luma also appears on Starship Mario and tells Mario about Co-Star Mode.

If the Co-Star Luma is there when Mario collects a Power Star or a Grand Star, it will appear along with Mario in the "You Got a Star/Grand Star!" victory pose.

When the Co-Star Luma is present, orange butterflies will appear in some galaxies. When the Co-Star Luma stops and spins on the butterflies, the butterflies will release 3 Star Bits and will disappear afterwards.